Marijuana is the most commonly used illicit drug in the United States. A new study reveals that Mariuana affects the brain, even with minimal use.

Marijuana use is associated with problems with motivation, attention, learning, and memory. Animal studies have shown that marijuana’s active ingredient, Δ9-tetrahydrocannabinol, produces structural changes in the nucleus accumbens. However, less is known about how low to moderate marijuana use affects brain structure in people, particularly in teens and young adults.

Researchers used magnetic resonance imaging (MRI) to compare the brains people 18-25 years old who reported smoking marijuana at least once per week with those with little to no history of marijuana use. Imaging data revealed significant brain differences between non users of marijuana and minimal users of marijuana.

Persons who were dependent on marijuana were excluded from the study. The size and shape of the amygdala (which is responsible for regulation of emotions) and the nucleus accumbens (which is responsible for motivation and reward processing) was different in casual or minimal users of marijuana compared to those who have not used. The scientists found that the more the marijuana users reported consuming, the greater the abnormalities in the nucleus accumbens and amygdala. The shape and density of both of these regions also differed between marijuana users and non-users. The study was published April 16 in The Journal of Neuroscience.

The findings suggest that marijuana affects the brain, even with minimal use. “This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy,” said Carl Lupica, PhD, who studies drug addiction at the National Institute on Drug Abuse, and was not involved with this study. “These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users.”